enhance(ui): tint toast backgrounds by level (#38919)

Toasts now use the same tinted backgrounds and borders as the flash
messages, replacing the solid full-color style. The first commit reverts
https://github.com/go-gitea/gitea/pull/38842, the second re-applies it
with tinting.

---------

Co-authored-by: silverwind <me@silverwind.io>

web_src/css/modules/toast.css

@@ -1,6 +1,5 @@
 .toastify {
   color: var(--color-text);
-  background: var(--color-menu);
   border: 1px solid var(--color-secondary);
   position: fixed;
   opacity: 0;
@@ -18,6 +17,26 @@
   opacity: 1;
 }
 
+.toast-success {
+  background: var(--color-success-bg);
+  border-color: var(--color-success-border);
+}
+
+.toast-info {
+  background: var(--color-info-bg);
+  border-color: var(--color-info-border);
+}
+
+.toast-warning {
+  background: var(--color-warning-bg);
+  border-color: var(--color-warning-border);
+}
+
+.toast-error {
+  background: var(--color-error-bg);
+  border-color: var(--color-error-border);
+}
+
 .toast-success .toast-icon { color: var(--color-success-text); }
 .toast-info .toast-icon { color: var(--color-info-text); }
 .toast-warning .toast-icon { color: var(--color-warning-text); }